There's been a spate of threads here, in General, & Road, about the possible perils of damaged carbon, fatigued aluminum, & such.

Some have responded that even the older high-performance steel race frames had their share of problems, too. Let's set aside crash damage, whatever the cause, or rust caused by neglect over the years. Sure, welds can fail, lugs can crack, (though I've never seen it personally), but really, weren't the bikes of 20-30+ years ago more durable and reliable than the seemingly planned obsolescence ones of today? Or are the specimens we have now just the survivors?

there is a simple metallurgical answer.
Endurance limit.
Steel has one, aluminum and carbon composite doesn't. Without mechanical or weathering damage to alter it, steel will last forever.
The endurance limit is a stress value below which no cumulative damage is formed in the material. The material will not crack.
Aluminum will eventually crack with enough cycles (millions, etc.) at any stress. Same with carbon composite, and there are many ways to produce undetectable matrix damage in composite.

An even simpler answer is that steel typically gives plenty of warning before it fails, if you look for the signs. Cracks in aluminum and carbon fiber propagate quickly.

current aluminum designs are crack-tolerant - they have to be. They are now designed with extra stiffness and redundant support, so a crack will unload as it grows, giving you a long time to find and repair it.

The evolution of CF use in bikes has come a long ways. But strangely there's a large differential between the properties to take abuse versus fatigue strength. Years ago, I crashed a few times on my early Kestrel CS-X mtn. bike. One was laughably a dang hard slam on a large rock and I went the other way. The dropout derailleur side, chainstay took a significant hit, chipped it but otherwise fine. NO way would some modern CF take something like that.

Let me remind, this was a FULL rigid, non-susp. fork atb bike. Think about it. The advent of full or front susp. encouraged a lighter, thinner layers and dope.

I'm not even sure an ally or steel frame would've taken the same hit without some major damage. I was impressed.

99.9% of bikes used today will never suffer from frame damage from reasonable use. Fit, ride quality and efficient performance is far more important than durability.

The caveat though (at least in my mind) is that you have to consider [MENTION=442798]bulldog1935[/MENTION]'s comment that "there are many ways to produce undetectable matrix damage in composite," or [MENTION=350383]crank_addict[/MENTION]'s statement that he "crashed a few times..." That makes me shy away from used CF bikes.

We have all seen steel bikes for sale with obvious front end damage. It's obvious because the crash pushes the fork back a bit, and you can often feel ripples under the down tube or see cracks in the paint. Even with that damage, many of those bikes can either be ridden as-is, or have their forks straightened, and maybe have their head tubes pulled back out. (My Medici is one such example.) Most of us have at least some seat-of-the-pants understanding of the properties of steel which make this possible. Where is the visible warning of a past crash in a carbon fiber frame?

and again, the problem is the use of "never"
Only steel qualifies here. Aluminum and composite both in normal use will eventually fail - almost 99% certain it won't be within your lifetime use - but undamaged, un-rusted steel can be handed down for generations.

To me, the scariest thing is people buying used bikes with carbon forks. If you buy an old frame with a carbon fork, you should probably buy a new fork.

Touchdown.

Many of us have a multitude of bikes in our stables.

Only three of mine will see more than a 1,000+ miles this season.

I hardly think durability will be an issue here.

what if your grandson takes it up

a friend in my riding group has a beautiful Lemond alloy frame with carbon fork that he bought used.
His rear wheel is going through sequential spoke pop - every time he installs a replacement spoke, the one next to it breaks on the next ride.
Probably needs a new wheelset/rebuild, but I would be more worried about the fork.

Nah, I wouldn't go that far.

If the bike has been raced, maybe.

Especially by a cat4.

So many of the CF bikes I've owned have been low mileage hook hangers.

My biggest concern is usually cleaning off the dust and the spider eggs from the previous owner's garage.

Hardly even broken in.......

My grandson won't likely be getting any of my bikes.

We are heading for a significant downsize within the next decade.

I've already promised to clear this place out when the time comes.

I wouldn't want to dump that burden on Mrs. Gomango.

Same goes for the rest of ocd related hobbies I have.

Carbon durability will be more of a problem when they've been around long enough that prices for used carbon bikes drop to the point where they're attractive to buyers looking for cheap run around bikes and who may have no interest or understanding of the characteristics of carbon frames. As we all know by now carbon is very strong when stressed the way it's designed to be stressed, but it doesn't do well when thrown into truck beds, dropped on concrete floors or having components overtorqued.

Literal Answers to Rhetorical Questions

Personal experience with steel frame failures:

1960 Capo Modell Campagnolo (db 531) -- downtube cracked behind head tube, just past butting taper, but frame had been bent back and straightened and was thus being ridden on borrowed time

1971 Nishiki Competition (db CrMo) -- seat tube lug broke off of bottom bracket shell at 40K mi / 20 year mark.

1973 Peugeot UO-8 (plain gauge carbon steel) -- chainstay cracked between the tire and chainring clearance dimples

Personal experience with steel rear axle failures (always drive side, adjacent to bearing cone):
1 solid French axle, 120mm OLD
1 hollow (QR) French axle, 120mm OLD
1 hollow (QR) Campag. axle, 126mm OLD, 25 years old

The thread title is clear and has been addressed. Although the other comments have validity based on what you do and how you ride. Racing will always push the limit and is not concerned about a lifetime away so time will tell what happens to bikes in current and recent use.

No one even mentioned titanium or the pressed material bikes like Griffen.

of course steel will fatigue and crack if its endurance limit is exceeded - the point is, it has one.

There are limits to design that can affect failures, and that envelope was often pushed for light weight.
There is also mechanical damage, wear, contact stress, binding, etc. that can increase operating stress above the endurance limit.

For an example of endurance I can throw out a 40,000-mi frame and 25,000-mi Zeus axle - I bought them both new.
I wasn't even worried cold-setting the rear triangles an extra 6mm. I wouldn't even think about setting it back.

There are defects in brazes that can eventually catch up. As mentioned before, once you add corrosion damage, all bets are off.
But again, we're dealing with a mature technology

One word:

Titanium (the other grey meat).

hardest part with titanium is joining it because of brittle constituents that form from the heat

That's why I'd stick to guys that make a LOT of titanium...but you don't hear of ti failure that often with the Moots, Eriksens, Sevens, etc.

I think they're wonderful, met a guy several times on one of my rides with a Ti LeMond and there are a few other great frame builders out there.
They're just out of my need and cost interest.
Regards

One need only look over in the 41 at the guys who are tired of their 18-speed Ultegra bikes and are looking to "upgrade" to 22 speed... most bikes will succumb to boredom or failing fitness or change of hobby, they will sit in the back of the garage and the dust will cake and the grease will caramelize and the hoods and tires will crack, until 30 years have gone and the family is moving Dad to memory care and the flippers declare it not worth their time to save.

My cheap 1990-ish Novara suffered a fork failure. My cheap aluminum 2002-ish Specialized has been subjected to far worse behavior, both riding and maintenance errors, and more miles over the years, and the only wear problem I ever had was the inner chain ring.

[MENTION=442798]bulldog1935[/MENTION], welcome to BF. I really appreciate your contributions.

[MENTION=129154]KonAaron Snake[/MENTION]: !

thank you Tom, I'm enjoying it here.

If one is uncomfortable with "new" tech don't buy it.

Since this is C&V by definition they ain't makin' em anymore.
Buying a used bike of any vintage/flavor is a Caveat Emptor evaluation, triple that for "race bikes".

Most bikes are not ridden enough to wear the mold marks off the tires and are highly unlikely to fail in service.

If you break your bike most likely you:

1) Did something exceptionally stupid, perhaps repeatedly.
b) Are a raging-watts-monster-mega-miles beast.

Congratulations either way,

-Bandera